Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 49

42. (Pg. 189) What determines the colour of any particular body?

43. (Pg. 189) What exemplifications are given?

44. (Pg. 189) By what reasoning is it proved, that bodies do not retain their colours in the dark?

45. (Pg. 190) What proof of the truth of this theory of colours, may be afforded by the prism?

46. (Pg. 191) Why will green leaves, when exposed to the red ray, appear of a dingy brown?

47. (Pg. 191) Bodies, in general, when placed in a ray differing in colour from their own, appear of a mixed hue, what causes this?

48. (Pg. 191) Why will bodies of a pale, or light hue, most perfectly, a.s.sume the different colours of the spectrum?

49. (Pg. 192) Upon what property in a body, does the darkness of its colour depend?

50. (Pg. 192) Why do some bodies appear white, others black, and others of different colours?

51. (Pg. 192) From what cause do blue articles appear green, by candle-light?

52. (Pg. 193) What is believed to be the cause, of the red appearance of the sun, through a fog, or misty atmosphere?

53. (Pg. 193) From what is the blue colour of the sky, thought to arise?

54. (Pg. 193) What would be the colour of the sky, did not the atmosphere reflect light?

55. (Pg. 193) From what cause do some bodies change their colour, as leaves formerly green, become brown, and ink, yellow?

56. (Pg. 194) Why is a black dress, warmer in the suns.h.i.+ne, than a white one of the same texture?

CONVERSATION XVII.

ON THE STRUCTURE OF THE EYE, AND OPTICAL INSTRUMENTS.

DESCRIPTION OF THE EYE. OF THE IMAGE ON THE RETINA. REFRACTION BY THE HUMOURS OF THE EYE. OF THE USE OF SPECTACLES. OF THE SINGLE MICROSCOPE.

OF THE DOUBLE MICROSCOPE. OF THE SOLAR MICROSCOPE. MAGIC LANTHORN.

REFRACTING TELESCOPE. REFLECTING TELESCOPE.

MRS. B.

The body of the eye, is of a spherical form: (fig. 1. plate 21.) it has two membranous coats, or coverings; the external one, _a a a_, is called the sclerotica, this is commonly known under the name of the white of the eye; it has a projection in that part of the eye which is exposed to view, _b b_, which is called the transparent cornea, because, when dried, it has nearly the consistence of very fine horn, and is sufficiently transparent for the light to obtain free pa.s.sage through it.

The second membrane which lines the cornea, and envelops the eye, is called the choroid, _c c c_; this has an opening in front, just beneath the cornea, which forms the pupil, or sight of the eye, _d d_, through which the rays of light pa.s.s into the eye. The pupil is surrounded by a coloured border called the iris, _e e_, which, by its muscular motion, always preserves the pupil of a circular form, whether it is expanded in the dark, or contracted by a strong light. This you will understand better by examining fig. 2.

_Emily._ I did not know that the pupil was susceptible of varying its dimensions.

_Mrs. B._ The construction of the eye is so admirable, that it is capable of adapting itself, more or less, to the circ.u.mstances in which it is placed. In a faint light, the pupil dilates so as to receive an additional quant.i.ty of rays, and in a strong light, it contracts, in order to prevent the intensity of the light from injuring the optic nerve. Observe Emily's eyes, as she sits looking towards the windows: the pupils appear very small, and the iris, large. Now, Emily, turn from the light, and cover your eyes with your hand, so as entirely to exclude it, for a few moments.

_Caroline._ How very much the pupils of her eyes are now enlarged, and the iris diminished! This is, no doubt, the reason why the eyes suffer pain, when from darkness, they suddenly come into a strong light; for the pupil being dilated, a quant.i.ty of rays must rush in, before it has time to contract.

_Emily._ And when we go from a strong light, into obscurity, we at first imagine ourselves in total darkness; for a sufficient number of rays cannot gain admittance into the contracted pupil, to enable us to distinguish objects: but in a few minutes it dilates, and we clearly perceive objects which were before invisible.

_Mrs. B._ It is just so. The choroid _c c_, is embued with a black liquor, which serves to absorb all the rays that are irregularly reflected, and to convert the body of the eye, into a more perfect camera obscura. When the pupil is expanded to its utmost extent, it is capable of admitting ten times the quant.i.ty of light, that it does when most contracted. In cats, and animals which are said to see in the dark, the power of dilatation and contraction of the pupil, is still greater; it is computed that the pupils of their eyes may admit one hundred times more light at one time than at another.

Within these coverings of the eye-ball, are contained, three transparent substances, called humours. The first occupies the s.p.a.ce immediately behind the cornea, and is called the aqueous humour, _f f_, from its liquidity and its resemblance to water. Beyond this, is situated the crystalline humour, _g g_, so called from its clearness and transparency: it has the form of a lens, and refracts the rays of light in a greater degree of perfection, than any that have been constructed by art: it is attached by two muscles, _m m_, to each side of the choroid. The back part of the eye, between the crystalline humour and the retina, is filled by the vitreous humour, _h h_, which derives its name from a resemblance it is supposed to bear, to gla.s.s, or vitrified substances.

[Ill.u.s.tration: PLATE XXII.]

The membranous coverings of the eye are intended chiefly for the preservation of the retina, _i i_, which is by far the most important part of the eye, as it is that which receives the impression of the objects of sight, and conveys it to the mind. The retina is formed by the expansion of the optic nerve, and is of a most perfect whiteness: this nerve proceeds from the brain, enters the eye, at _n_, on the side next the nose, and is finely spread over the interior surface of the choroid.

The rays of light which enter the eye, by the pupil, are refracted by the several humours in their pa.s.sage through them, and unite in a focus on the retina.

_Caroline._ I do not understand the use of these refracting humours: the image of objects was represented in the camera obscura, without any such a.s.sistance.

_Mrs. B._ That is true; but the representation became much more strong and distinct, when we enlarged the opening of the camera obscura, and received the rays into it, through a lens.

I have told you, that rays proceed from bodies in all possible directions. We must, therefore, consider every part of an object which sends rays to our eyes, as points from which the rays diverge, as from a centre.

_Emily._ These divergent rays, issuing from a single point, I believe you told us, were called a pencil of rays?

_Mrs. B._ Yes. Now, divergent rays, on entering the pupil, do not cross each other; the pupil, however, is sufficiently large to admit a small pencil of them; and these, if not refracted to a focus, by the humours, would continue diverging after they had pa.s.sed the pupil, would fall dispersed upon the retina, and thus the image of a single point, would be expanded over a large portion of the retina. The divergent rays from every other point of the object, would be spread over a similar extent of s.p.a.ce, and would interfere and be confounded with the first; so that no distinct image could be formed, and the representation on the retina would be confused, both in figure and colour. Fig. 3. represents two pencils of rays, issuing from two points of the tree, A B, and entering the pupil C, refracted by the crystalline humour D, and forming on the retina, at _a b_, distinct images of the spot they proceed from. Fig. 4.

differs from the preceding, merely from not being supplied with a lens; in consequence of which, the pencils of rays are not refracted to a focus, and no distinct image is formed on the retina. I have delineated only the rays issuing from two points of an object, and distinguished the two pencils in fig. 4. by describing one of them with dotted lines: the interference of these two pencils of rays on the retina, will enable you to form an idea of the confusion which would arise, from thousands and millions of points, at the same instant pouring their divergent rays upon the retina.

_Emily._ True; but I do not yet well understand, how the refracting humours, remedy this imperfection.

_Mrs. B._ The refraction of these several humours, unites the whole of a pencil of rays, proceeding from any one point of an object, to a corresponding point on the retina, and the image is thus rendered distinct and strong. If you conceive, in fig. 3., every point of the tree to send forth a pencil of rays, similar to those from A B, every part of the tree will be as accurately represented on the retina, as the points _a b_.

_Emily._ How admirably, how wonderfully, is this contrived!

_Caroline._ But since the eye absolutely requires refracting humours, in order to have a distinct representation formed on the retina, why is not the same refraction equally necessary, for the images formed in the camera obscura?

_Mrs. B._ It is; excepting the aperture through which we receive the rays into the camera obscura, is extremely small; so that but very few of the rays diverging from a point, gain admittance; but when we enlarged the aperture, and furnished it with a lens, you found the landscape more perfectly represented.

_Caroline._ I remember how obscure and confused the image was, when you enlarged the opening, without putting in the lens.

_Mrs. B._ Such, or very similar, would be the representation on the retina, una.s.sisted by the refracting humours.

You will now be able to understand the nature of that imperfection of sight, which arises from the eyes being too prominent. In such cases, the crystalline humour, D, (fig. 5.) being extremely convex, refracts the rays too much, and collects a pencil, proceeding from the object A B, into a focus, F, before they reach the retina. From this focus, the rays proceed, diverging, and consequently form a very confused image on the retina, at _a b_. This is the defect in short-sighted people.

_Emily._ I understand it perfectly. But why is this defect remedied by bringing the object nearer to the eye, as we find to be the case with short-sighted people?

_Mrs. B._ The nearer you bring an object to your eye, the more divergent the rays fall upon the crystalline humour, and consequently they are not so soon converged to a focus: this focus, therefore, either falls upon the retina, or at least approaches nearer to it, and the object is proportionally distinct, as in fig. 6.

_Emily._ The nearer, then, you bring an object to a lens, the further the image recedes behind it.